present invention relates to a system for setting up a channel in a portable telephone system using portable telephone sets, which are connected to a general public telephone network, a private branch exchange network, etc. and which can be carried to any place and are capable of originating and receiving a call.
At present a variety of mobile communications are in practical use in many countries. Of them, telephones primarily for voice transmission are a domestic maritime mobile radio telephone, a land mobile radio telephone, a cordless telephone, and a section party telephone within a station yard. These various mobile communications are respectively operated as independent systems; for instance, a cordless telephone could not start a call when it is brought in a car. This is evident from the fact that the cordless telephone and the land mobile radio telephone differ from each other in many points; for example, they use different working frequencies and different control methods for their control channels, and in addition, the input radio wave field strength necessary for the receiver of the called land mobile radio telephone would not be satisfied with the transmitting power of the cordless telephone.
Further, the cordless telephone generally comprises a portable telephone set and a key service unit which are connected thereto by way of a radio communication circuit and to an exchange by way of a wire communication circuit. In addition to this, the telephone set and the key service unit usually form a specific relationship to each other. The telephone set cannot communicate with a key service unit other than the specified one even if they happen to lie at a short distance over which radio communication could be established between them. Similarly, the key service unit is not permitted to communication with other telephone sets than the specified one.
This is a matter of course in some sense. It is undesirable if a portable telephone set and a key service unit which have no particular relationship to each other are connected without any conditions, because this will incur the possibility of mischarging or intentional shifting of a charge due on the connected telephone set to an unconnected one.
People have long been waiting eagerly for the appearance of a portable telephone system which technically overcomes the above-noted difficulties and enables a person carrying the portable telephone set to make a call from any place to any place at any time and to receive a call wherever he may go.
Even in a society like our country where the number of telephone terminals for public communication use is now as large as more than forty millions and where public telephone sets are accessible everywhere on the streets, not to speak of land mobile radio telephones on cars, train public telephones on Shinkansen trains, and domestic maritime radio telephones on ships, there still exists a strong demand for such a portable telephone system for the reason given below.
This is because everyone wishes to have a portable telephone for his exclusive use. That is to say, it would be extremely convenient if a person carrying the portable telephone could freely use it just like a telephone installed at his home, for there are cases where he feels askward, for some reason, to ask for permission to use a telephone at other people's office or home, where no public telephone can be found on the streets, where anyone does not happen to have small money or a telephone card with him, where no public telephones installed in front of stores or the like can be used late at night, and where it is troublesome to look for a public telephone at a crowded place like a ballpark or area for special events.
The same applies to receiving a call. It would be far more convenient than in the above cases if anyone carrying the portable telephone could receive a call from any place at any time wherever he may go. This is because although anyone can make a call from a nearby public telephone at any time it is unpredictable when he will be called while he is away from his home or office. Moreover, it is impossible, with the conventional telephone system, that two persons communicate with each other if they are moving in different places.
In addition, the portable telephone will enable an emergency or urgent call to be made more simply and more quickly than the systems now employed. Besides, it would be more convenient if the portable telephone set could be used for opening a credit (a credit function) or as a substitute for an identification card.
As described above, there is eager demand for the realization of such an ultimate portable telephone even at present.
To meet this demand, various systems have been proposed which are, however, unsatisfactory in many respects.